WASHINGTON — NASA has ordered some last-minute tests on the space shuttle Discovery's giant fuel tank to see if the spacecraft is safe to blast off later this month.

Discovery is slated to launch toward the international space station on Aug. 24, but only if its fuel tank passes new round of pull tests to make sure that foam insulation in certain areas won't peel off and damage the orbiter during launch, mission managers said Thursday. An Aug. 24 launch would lift off at about 2 a.m. ET, if approved.

NASA shuttle program manager John Shannon said engineers this weekend will pull samples from part of the 15-story tank called the "intertank" - a ribbed, barrel-like area just above the mid-point of the tank. An unusually high amount of foam fell from a similar area during the launch of the shuttle Endeavour last month and NASA is not sure why.

After Endeavour's launch, engineers performed an initial round of pull tests on the backside of Discovery's tank and found the foam insulation in good health. The new tests will check regions on the front of the tank to be sure foam there won't pop free and damage Discovery, Shannon said.

Engineers are also using a special X-ray machine to scan a series of ice-frost ramps, essentially foam-covered brackets, on the next shuttle fuel tank to fly after Discovery's current mission. Some foam popped off a similar ice-frost ramp during Endeavour's launch and NASA wants to be sure there are no generic flaws with the fuel tanks before clearing Discovery for flight.

"We have a lot of confidence that we're in good shape," Shannon told reporters Thursday. Aside from the extra fuel tank tests, Discovery is in fine shape for its planned launch later this month, he added.

Foam tests ahead
NASA has kept a close watch on foam debris during shuttle launches since a piece of foam punched a hole in the wing-mounted heat shield of the shuttle Columbia, leading to its destruction in 2003. Seven astronauts were killed in the disaster.

Shannon and other top NASA officials will review the results from this weekend's fuel tank tests next week during a standard flight readiness review meeting. By then, mission managers should know whether Discovery can fly as is or be hauled back inside NASA's cavernous Vehicle Assembly Building at the Kennedy Space Center in Florida for repairs.

If any repairs are needed, they could delay Discovery's flight beyond its August window to mid-September or Oct. 17, Shannon said.

NASA hopes to launch Discovery by the end of August in order to fly the mission before the planned Sept. 10 launch of Japan's first unmanned cargo ship to deliver supplies to the space station.

A Russian unmanned cargo ship is also due to depart the station next month to set the stage for the arrival of a Soyuz spacecraft carrying new crewmembers and space tourist to the station. That Soyuz is slated to launch Sept. 30, adding more space traffic to the mix.

"So it's a really busy time for the space station," said Kirk Shireman, NASA's deputy space station program manager.

Fresh supply run
Discovery is slated to launch seven astronauts on a 13-day mission to the space station to replace one member of the outpost's crew and deliver vital supplies and new equipment to the orbiting laboratory. The shuttle will carry a cargo pod packed with new science racks, fresh food, a new astronaut sleeping berth and a treadmill named COLBERT in honor of American comedian Stephen Colbert.

Three spacewalks are also planned during the mission to perform station maintenance and replace a massive ammonia tank for the station's cooling system. The tank weighs as much as a small car and will require two spacewalks to replace.

The mission is only the second time in history that 13 people, seven from Discovery and the station's core six-person crew, will be aboard the orbiting laboratory at the same time. Aside from a broken American-built oxygen generator, which astronauts are expected to fix in the next few weeks, the station is ready for its second shuttle visit in two months, Shireman said.

Veteran shuttle commander Rick Sturckow will lead the mission, which will replace NASA astronaut Tim Kopra aboard the station with fellow American astronaut Nicole Stott.

Discovery's flight will mark NASA's fourth shuttle mission of up to five planned for this year. NASA plans to launch seven more shuttle missions, including Discovery's, to complete space station construction by 2010, when the aging three-shuttle fleet is slated to retire.

A blue-ribbon panel appointed by the White House is currently reviewing NASA's plan to retire the shuttles by 2010 and replace them with new spacecraft aimed at returning astronauts to the moon by 2020. One of the proposals reviewed by the committee calls for a more heavily shuttle-derived vehicle and extending shuttle missions for several years beyond 2011.

Shannon said any decision to continue flying the space shuttle beyond the final seven missions, even at a rate of one to two shuttle flights per year, must be made by President Barack Obama's administration before the end of this year. NASA currently plans to lay off some 1,200 people — 10 percent of the shuttle workforce — and would have to renew some contracts since the last external tank and shuttle main engine test, for example, have already been performed.

"We're getting to the point, if we don't have a decision late this year, we're going to end up having a gap between when we would stop flying the flights that are currently on the manifest and any new flights that would be out there," Shannon said. "So it's getting very late in the game for us."